Ion-enhanced thermoelectric generator

ABSTRACT

A thermoelectric converter including a thermoelectric generator and a radiation source. The thermoelectric generator includes a hot source, a cold source, n-type material, and p-type material. The radiation source emits ionizing radiation that increases electrical conductivity. Also detailed is a method of using radiation to reach high efficiency with a thermoelectric converter that includes providing a thermoelectric generator and a radiation source, with the thermoelectric generator including a hot source, a cold source, n-type material, and p-type material, and emitting ionizing radiation with the radiation source to increase the electrical conductivity which strips electrons in the n-type material, the p-type material, or both the n-type material and p-type material from their nuclei with the electrons then free to move within the material.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims the benefit of priority to U.S. ProvisionalApplication No. 63/076,187, filed Sep. 9, 2020, and to U.S. ProvisionalApplication No. 63/150,525, filed Feb. 17, 2021, each of which isincorporated herein by reference in their entirety.

GOVERNMENT STATEMENT

The invention was made by an agency of the United States Government orunder a contract with an agency of the United States Government. Thename of the U.S. Government agency and Government contact number are:NASA (Shared Services Center, NSSC), Contact No. 80NSSC19K0962.

TECHNICAL FIELD

The present disclosure relates generally to electric generation andspecifically to systems and methods of augmented thermoelectricgeneration.

BACKGROUND

Thermoelectric generators may be useful for applications requiring nomoving parts or particularly long lifetimes. NASA's multi-missionradioisotope thermoelectric generator (MMRTG) and their enhanced MMRTG(eMMRTG) have efficiency levels between 5.5%-7.4% at the beginning oftheir life. A dynamic cycle may achieve efficiencies approaching 20-25%but rely on working fluids and moving machinery. While increasing thetemperature gradient would also increase efficiency, this option islimited in its ability to change the efficiency due to limitations ofmaterials and heat rejection systems. Although thermoelectric generationhas been used for space exploration, waste heat conversion, andinstrument cooling, the low conversation efficiency prevents morewidespread applications.

SUMMARY

Aspects and applications presented herein are described below in thedrawings and detailed description of the exemplary embodiments shown.Unless specifically noted, it is intended that the words and phrases inthe specification and the claims be given their plain, ordinary, andaccustomed meaning to those of ordinary skill in the applicable arts.

In the following description, and for the purposes of explanation,numerous specific details are set forth in order to provide a thoroughunderstanding of the various aspects of the exemplary embodiments shown.It may be understood, however, by those skilled in the relevant arts,that the presented exemplary embodiments may be practiced without thesespecific details. In other instances, known structures and devices areshown or discussed more generally in order to avoid obscuring theexemplary embodiments shown. In many cases, a description of theoperation is sufficient to enable one to implement the various forms ofthe exemplary embodiments shown, particularly when the operation is tobe implemented in software. It should be noted that there are manydifferent and alternative configurations, devices and technologies towhich the disclosed exemplary embodiments shown may be applied. The fullscope of the exemplary embodiments shown are not limited to the examplesthat are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a system with augmented thermoelectric generators coupledwith a radiation source.

FIG. 2 illustrates a chart showing effects of radiation on electricalconductivity.

FIG. 3 illustrates ZT values of various thermoelectric materials atvarious temperatures.

FIGS. 4A through 4F illustrate penetration depth and the ionizationprofile of different alpha particle sources within a SiGe TEG foot

FIGS. 5A and 5B show the difference between a 241Am source and a10B(n,α) source and its corresponding change in the volume influenced byradiation induced conductivity (RIC).

FIG. 6 illustrates a chart showing statistical distribution of RICinfluence volume coverage within a PbTe matrix.

FIG. 7 illustrates a chart showing particle size, alpha source type, andATEG foot type effect on RIC vol % infill fraction.

FIG. 8 illustrates a chart showing predicted PbTe enhanced efficiencydue to alpha sources within its material matrix.

FIG. 9 illustrates a chart showing figures of merit associated with theATEG created from PbTE materials with the alpha source particles.

FIG. 10 shows conductivity multiplier plotted against change in figureof merit and the efficiency of a PbTe—PbTe ATEG.

FIGS. 11A and 11B illustrate charts showing the performance of a PbTeATEG under various temperature gradients.

FIGS. 12A through 12F illustrate charts showing performancecharacteristics of an ATEG system that produces 3 kW of electricalpower.

FIGS. 13A through 13F illustrate COMSOL simulations of a bismuthtelluride (Bi2Te3) p-type and n-type ATEG with applied conductivitychanges ranging from 5 to 100.

FIG. 14 illustrates a chart showing Boron Nitride resistance effects dueto neutron flux from nuclear reactor.

FIG. 15 illustrates a chart showing Boron Nitride complete test up to ½Mega-Watt (MW) power level observing ˜50 times conductivity multiplier.

FIGS. 16A through 16I illustrate COMSOL modeling results for BNthermoelectric junction for conductivity multipliers of 1, 30, and 50×.

FIG. 17 illustrates a chart showing Seebeck coefficient of boron basedthermoelectric materials.

FIG. 18 illustrates a chart showing response of electrical conductivityof showing boron based thermoelectric materials.

FIGS. 19A through 19C illustrate charts showing computational modelingresults for a rare earth boron based ATEGs at different levels of RIC.

FIG. 20A illustrates a SPEAR reactor with heat pipes, moderators,reflectors, control rod, and low enriched uranium fuel.

FIG. 20B illustrates layout of SPEAR reactor.

FIGS. 21-23 illustrate portable SPEAR power source.

FIG. 24 illustrates SPEAR spacecraft with CubeSat swarm payload.

FIG. 25 illustrates electrical subsystem of SPEAR.

DETAILED DESCRIPTION

In one aspect, the embodiments detailed herewithin, apply to anythermoelectric converter or Peltier cooler which utilizes ionizingradiation to increase performance. The exemplary devices as describedherein can be made of any material that allows them to function as powerconversion systems, and may include a variety of different radiationsources.

FIG. 1 illustrates system 100 comprising augmented thermoelectricgenerators (ATEGs) 110 a-110 n coupled with radiation source 120, inaccordance with a first embodiment.

ATEGs 110 a-110 n comprise hot side 130, cold side 140, n-type material150, and p-type material 160. In one aspect, radiation source 120 emitsionizing radiation (e.g. alpha, beta, gamma, or neutrons), whichincreases the electrical conductivity (including, e.g., n-type material150 and p-type material 160) as particle interactions strip electrons inthe material from their nuclei, and the affected electrons are free tomove within the material. Additionally, FIG. 1 shows a berylliumreflector 10, lithium hydride moderator material 20, boron carbidecontrol rod 30, heat pipe contact area 40, uranium fuel 50, heat pipelocations 60 (shown as holes in FIG. 1), and Ion Enhanced ThermoelectricGenerator array 110 a-110 n.

Thermoelectric conversion may be used for space exploration, waste heatconversion, and a method to cool instruments. The advancedthermoelectric generator (ATEG) increases current thermoelectricgenerator (TEG) efficiencies by two to three times. As described infurther detail herewithin, ATEGs may be used to produce power for deepspace probes without the need for large solar panels or costlyradioisotope thermoelectric generator (RTG) power sources. Additionally,or optionally, exemplary embodiments further detail mobile powergenerators coupling ATEGs to neutron power sources.

TEGs function by utilizing the Seebeck effect to change the distributionof electrons within a material due to a heat source applied at one end.This heat source pushes electrons away and towards the cold side, wherethey build up and create a voltage. By using two different materials inparallel, two different voltages can be achieved, and the differencebetween these voltages forms a potential that can be used to create anelectric circuit, and thus electric power. In addition, if theelectrical power is applied to leads of ATEGs 110 a-110 n, a temperaturedifferential is formed on the plates, and the exemplary device caninstead provide cooling. This arrangement creates a type of cooler,called a “Peltier cooler”, which can also benefit from the augmentationdisclosed in exemplary embodiments detailed herein.

FIG. 2 illustrates chart 200 showing effects of radiation on electricalconductivity, according to measurements of the conductivity change inmaterials exposed to radiation fields in a study by Oak Ridge NationalLaboratory (additional details available at Shikama, T., et. al.“Electrical properties of ceramics during reactor irradiation,” Journalof Nuclear Materials 258-263 (1998), https://www.sciencedirect.com/science/article/abs/pii/S0022311598003006). FIG. 2 shows electricalconductivity components in fine-grained 99.99% purity alumina cable(“CR125”). In FIG. 2, electrical conductivity values as a function oftemperature for the sample when irradiated with a dose of 42 Gy/s isshown at 202, electrical conductivity values as a function oftemperature for the sample when irradiated with a dose of 7 Gy/s isshown at 204, electrical conductivity values as a function oftemperature for the sample when irradiated with a dose of 0.7 Gy/s isshown at 206, and electrical conductivity values as a function oftemperature for the sample when not irradiated at all is shown at 208.The major change in conductivity can be seen to occur at relatively lowtemperatures (100-200° C.) and at radiation levels of 0.7 Gy/s or aboveas shown at 206. The produced radiation level varies based on thematerial used, but for strontium the radiation level translates roughlyto 0.14 grams of radioisotope per 100 grams of thermoelectric material,or 0.14% of the total mass.

A ZT value is used as the performance metric to characterize theproperties of a thermoelectric material. The ZT value of a singlethermoelectric material is given by Equation 1:

$\begin{matrix}{{zT} = \frac{S^{2}T}{\frac{\kappa}{\sigma}}} & (1)\end{matrix}$

wherein, σ is the electrical conductivity, S is the Seebeck Coefficient,and K is the thermal conductivity of a material at temperature T. Alarger value of ZT indicates a higher performing thermoelectricmaterial. From this equation, for a constant value of the SeebeckCoefficient, the ZT value increases proportionately for (1) increases inelectrical conductivity, (2) increases in temperature, or (3) decreasesin thermal conductivity.

A thermoelectric generator may comprise two materials, such as, forexample, a first material comprising a p-type semiconductor and a secondmaterial comprising an n-type semiconductor. The performance of TEGsdepends heavily on the ZT value. For a thermoelectric generatorcomprising a p-type semiconductor and an n-type semiconductor, is givenby Equation 2:

$\begin{matrix}{{Z\overset{\_}{T}} = \frac{\left( {S_{p} - S_{n}} \right)^{2}\overset{\_}{T}}{\left( {\left( \frac{\kappa}{\sigma_{o}} \right)^{\frac{1}{2}} + \left( \frac{\kappa_{n}}{\sigma_{n}} \right)^{\frac{1}{2}}} \right)^{2}}} & (2)\end{matrix}$

wherein, σ is the electrical conductivity, S is the Seebeck Coefficient,and K is the thermal conductivity of a p-type semiconductor and a is theelectrical conductivity, S is the Seebeck Coefficient, and K is thethermal conductivity of an n-type semiconductor and the temperature, isthe average temperature between hot side 130 and cold side 140.

The figure of merit increases in response to increasing one or more ofthe electrical conductivity, average temperature, and the Seebeckcoefficient and/or decreasing the thermal conductivity. The maximumtheoretical conversion efficiency of the thermoelectric generator may becalculated according to Equation 3:

$\begin{matrix}{\eta = {\frac{\Delta\; T}{T_{h}}\frac{\sqrt{1 + {Z\overset{\_}{T}}} - 1}{\sqrt{1 + {Z\overset{\_}{T}}} + \frac{T_{c}}{T_{h}}}}} & (3)\end{matrix}$

wherein, the conversion efficiency 11 is dependent on hot sidetemperature Tb and the cold side temperature T_(c). As ZT approachesinfinity, the conversion efficiency approaches the Carnot limit,

$\frac{\Delta T}{T_{h}},$

which is the theoretical maximum.

Many TEGs have a ZT value around 1, as seen in FIG. 3. A ZT value around1 usually translates to a relatively poor efficiency, as current TEGshave efficiencies around 6-8%.

FIG. 3 illustrates ZT values of various thermoelectric materials atvarious temperatures. Using increases in electrical conductivity modeledas 30× due to a 0.7 Gy/s dose, a model was generated to calculateefficiency using a TEG material with a ZT of 0.5, a hot side temperatureof 200° C., a cold side temperature of 25° C. From this exemplaryembodiment, the ratio of efficiencies could be found between theunmodified TEG and the enhanced TEG. The increase in efficiency was over535%, increasing from 4.4% to 23.7% with the modification.

In one embodiment, a TEG having a mass of 48 g, with dimensions 56 mm×56mm under the stated conditions provides a power level of 9 Watts.Applying the calculated efficiency increase, the 48 g TEG unit insteadprovides 48.15 Watts of power with 34 mg of strontium dopant.

The inclusion of a small amount of alpha or beta producing radioisotopehoused safely within the feet of the P-N junction of the TEG emitsradiation that improves the performance of the TEG. However, outsideradiation sources, such as, for example, accelerators or existingradiation fields, may be used to introduce the ionizing particles. Theionizing radiation emitted by the radiation source excites and ionizeselectrons in the material, which increases the electron mobility withinthe P-N junction feet, which increases the performance of the ATEGs. Theperformance is particularly significant at lower temperatures whereefficiency would have been relatively lower.

Although in at least one exemplary embodiment detailed herein athermoelectric generator is described as comprising a first materialcomprising a p-type semiconductor and a second material comprising ann-type semiconductor, alternative embodiments include a variety ofmaterials capable of meeting the parameters detailed herein and couldinclude additional p-type semiconductors or n-type semiconductors.

Advanced Thermoelectric Generators

In order to increase the figure of merit of thermoelectric generators tooutpace the performance of current deep space and terrestrial powergeneration systems, in at least one aspect, the presented exemplaryembodiments utilize the phenomena known as radiation inducedconductivity (RIC). In this process, ionizing radiation causes atoms inthe target material to ionize and free electrons, which increases theelectrical conductivity of the material. These electrons eventuallyreturn to their host nucleus, returning the electrical conductivity ofthe material to its original state. Radiation can be continuouslyapplied to re-ionize the atom and maintain a constant state of elevatedelectrical conductivity.

The ATEG units function by irradiating the feet of a thermoelectricgenerator and modifying the material properties of the feet. A source ofionization must be present for the ATEGs to function, which can beaccomplished through two different methods: a radioisotope source orneutron interactions. Both methods result in ATEG units having thepotential to reach comparable efficiency and power production as atraditional dynamic power cycle, but without utilizing moving parts.

Radioisotope Source

A common source of ionizing radiation is from radioisotopes, whichcontinuously emit ionizing radiation. Often this is in the form of analpha particle, which has two protons and two neutrons, and ispositively charged. Common radioisotopes include, plutonium-238,americium-241, and polonium-210. The energy of the alpha particledepends on the source and may typically exceed 5 Mega Electron Volt(MeV). Depending on the ionization energy of the foot material, thesealpha particles can ionize hundreds of thousands of atoms, and eachradioisotope particle can produce billions of alpha particles per seconddepending on its mass. With a properly designed radioisotope dopant, acontinuous supply of ionized atoms and free electrons may be availablewithin the ATEG to maintain the elevated electrical conductivity for thelifetime of the radioisotope.

Hot pressing or spark plasma sintering (SPS) are used to embed the alphasource particles within the ATEG materials. The hot press processcomprises heating and compressing powders of the ATEG material and thealpha source particles until they are densified into a solid object. TheSPS process is similar but additionally comprises introduction of anelectric current. Depending on the material, either hot pressing or SPSmay be used. Radioisotope particles would simply be mixed into thepowder and pressed down into a usable ATEG foot.

This type of radiation source would provide a stand-alone ATEG thatwould operate with higher efficiencies in accordance with its half-life.Alpha particles require little in the way of shielding, making itsuitable for most applications that require solid state power productionfrom a heat source. ATEGs could be paired with traditionalheat-to-electric systems to increase their power density and expandtheir mission capabilities with virtually no penalty.

Neutron Interaction

As disclosed herein, the radiation source may comprise a neutron source.When a neutron is captured by an atom, that atom undergoes transmutationand, if the resultant nuclide is unstable, it may decay. In thisspecific case, boron-10 (10_(B)) was used to produce alpha particlesthrough neutron absorption. Boron is an abundant element in manythermoelectric materials and semi-conductor dopants which alsoadvantageously possesses a large neutron capture cross section forthermal neutrons. Natural boron comprises approximately 80% 11_(B) andapproximately 20% 10_(B). When a 10_(B) atom captures a neutron, itreleases a 2.5 MeV alpha particle and decays into lithium-7. Thereleased alpha particle penetrates into the ATEG foot, thus ionizing thematerial and freeing electrons to increase the electrical conductivity,similar to the dopant radioisotope sources disclosed herein.

A 10_(B) atom has many advantages over radioisotopes, including that: itis not inherently radioactive, it is safer to handle, and it is notregulated by the Nuclear Regulatory Commission (NRC). This allows fortesting and validation of the ATEG without the need to obtainradioisotopes. Boron loaded materials were tested at the Kansas StateTRIGA reactor to investigate the RIC of a potential thermoelectricmaterial. Results showed a drastic increase in conductivity as predicted(see FIG. 15 and/or additional information can be found at: T. M. Howe,S. D. Howe, J. Miller Novel Deep Space Nuclear Electric PropulsionSpacecraft Nuclear and Emerging Technologies for Space (NETS), 2020).

ATEG Modeling

Investigation into the ATEG was performed to predict the performance ofthe ATEG with a large variety of inputs. As stated hereinabove, the mosteffective method to increase the efficiency of TEGs is to change theelectrical conductivity of the sample. Modification of the Seebeckcoefficient and thermal conductivity due to radiation exposure showedpositive results as well. However, these factors would not pose assignificant changes to the ATEGs efficiency as altering the material'selectrical conductivity. Previous studies have shown several orders ofmagnitude difference in electrical conductivity, while changes inthermal conductivity and Seebeck coefficient are limited to only a fewfactors difference.

Augmented thermoelectric generators (ATEGs) thermocouple may be embeddedwith RIC dopant particles. The RIC particles have their own thermalconductivity and electrical conductivity separate from the matrix. Thematrix also has its own thermal and electrical conductivity separatefrom the RIC particles. The area at 105, 106, and 107 in FIG. 1 is theRIC influenced area which may see localized increases in electricalconductivity greater than the surrounding matrix. FIG. 1 alsoillustrates how RIC influence areas can overlap forming a pathwaythrough the material with greater electrical conductivity.

A key component of the model was to predict the amount of RIC dopant(radioisotope, or neutron interaction) that needed to be added to thethermoelectric material to increase its electrical conductivity toadequate levels. The volume percentage (vol %) and particle size of theRIC dopant directly impacts the electrical conductivity of the sample.Alpha particles are emitted and penetrate into the matrix which causeslocalized regions of increased conductivity shown at 105, 106, and 107in FIG. 1. These regions of localized conductivity can overlap,increasing the amount of ionizations these areas. This further increasesthe electrical conductivity in that area as well. Multiple areas ofoverlapping RIC influenced areas connect forming a highly conductivepathway through the TEG foot. Addition of RIC dopant particles mayaffect the thermal conductivity, Seebeck coefficient, and electricalconductivity. A primary concern with more thermally conductivityparticles, is to avoid forming a pathway through the ATEG foot of highthermally conductive particles, which would reduce the ATEGs efficiency.However, because of the volume of material under RIC influence isgreater than the particle vol % this may be unlikely to occur. Areasunder the influence of the ionizing radiation are also expected toexperience decreases in thermal conductivity and increases in Seebeckcoefficient. However, these changes were overshadowed by the moredominating change in electrical conductivity.

TABLE 1 NiO SiGe Bi₂Te₃ PbTe Material (6.67 g/cc) (3.01 g/cc) (7.7 g/cc)(8.16 g/cc) ²³⁸Pu (5.6 MeV) 12.4 26.2 20.3 19.7 ²⁴¹Am (5.5 MeV) 12 25.519.7 19.2 ¹⁰B (2.5 MeV) 4.46 8.96 7.18 7.06

TABLE 1 shows alpha particle penetration depth in various thermoelectricgenerator materials. Penetration depth is directly related to volume ofmaterial effected by the radiation induced conductivity. The averagealpha particle energy is taken from each source and all penetrationdepths are represented in microns. The first column in TABLE 1 is theradioisotope material that emits alpha particles with the energies inparenthesis. The next four columns are the matrix materials that thealphas travel through, and the values are how far they go.

FIG. 3 shows ZT values of some common TEG materials. In FIG. 3, ZTvalues of Sb2Te3 as a function of temperature are shown at 302, ZTvalues of PbTe as a function of operating temperature are shown at 304,ZT values of PbTeSe as a function of operating temperature as shown at306, ZT values of TAGS as a function of operating temperature are shownat 308, ZT values of CeFe4Sb12 as a function of operating temperatureare shown at 310, ZT values of PbTe as a function of operatingtemperature using data from designs in the 1960s are shown at 312, ZTvalues of Yb14MnSb11 as a function of operating temperature are shown at314, and ZT values of SiGe as a function of operating temperature areshown at 316.

FIGS. 4A through 4F show a simulation of how alpha particles penetrateinto materials. The images (402, 404, 406) of FIGS. 4A, 4B, 4C showalpha particles from plutonium, americium, and boron in silicongermanium. Plutonium and americium have deeper penetration depths due tothe higher energy alpha particles emitted. The images (408, 410, 412) ofFIGS. 4D, 4E, 4F track the number of ionizations in the matrix material(414, 418, 422) and energy lost per angstrom traveled (416, 420, 424) ofthe alpha particle. FIGS. 4A through 4F illustrates penetration depthand the ionization profile of different alpha particle sources within aSiGe TEG foot, according to an exemplary embodiment. Higher energy alphaparticles penetrate further into the material, resulting in larger areasunder RIC influence.

The radiation sources described previously all have different alphaparticle energy levels, and therefore penetrate to different depthswithin the thermoelectric material. This behavior is visible in TABLE 1for some common thermoelectric materials showing the numbers forpenetration depth are different for each material/alpha emitter. Boron,with the smallest alpha particle energy, has roughly ⅓ the penetrationdepth of the two radioisotope sources studied, which would require it tohave a higher RIC dopant vol % to reach the same change in electricalconductivity. FIGS. 4A through 4F show how the energy of the alphaparticle effects the penetration depth and the number of ionizationswithin a SiGe matrix. A larger penetration depth can also aid inreducing the amount of radioisotope used per ATEG.

FIGS. 5A and 5B show the difference between a 241Am source and a10B(n,α) source and its corresponding change in the volume influenced byRIC. FIG. 5A shows a 241Am RIC dopant within a SiGe matrix and FIG. 5Bshows 10B(n,α) particles within a SiGe matrix. In order to reach thesame RIC influence vol % a larger amount of 10B must be used, as thegoal is to completely encompass the material under RIC influence. Charts502 and 504 showing particle distribution and RIC penetration, accordingto an exemplary embodiment.

Particles 510 of randomly distributed Am-241 of chart 502 and particles520 of randomly distributed B-10 of chart 504 penetrate to depths due to5.49 MeV alpha particles and 2.5 MeV particles. In order to cover thesame vol % as the 241Am, larger vol % of the 10B(n,α) may be required.

Decreasing the RIC dopant particle sizes also has a significant effecton the electrical conductivity of the samples. If small particle sizesare used, a larger volume of the ATEG foot may be under RIC influencefor the same vol %. Larger particles may result in smaller vol % underRIC influence as the particles may not be as dispersed within thematrix. In samples that contain (n,α) particles this holds true as well,however, particles such as 10_(B) may already be atomisticallydistributed within the material because it is one of the elements thatmake up the compounds. This should yield much higher vol % under RICinfluence as compared to materials made through sintering with RICparticles.

As shown in FIG. 5, areas within the matrix have overlapping areas ofRIC influence. These areas may see increases in electrical conductivitycompared to other areas depending on the RIC influence areas that areoverlapping. This also means that for each vol % infill and particlesize distribution there may be a statistical distribution of RICcoverage that can be expected. This provides valuable information inunderstanding how much radioisotope of (n,α) must be added to the ATEGfoot to achieve the required change in electrical conductivity. Thesestatistical distributions also aid in determining the performance ofpotential ATEG materials without the need to run countless simulationsbased on their material properties.

FIG. 6 illustrates chart 600 showing statistical distribution of RICinfluence volume coverage within a PbTe matrix, according to anexemplary embodiment. Smaller particle sizes result in larger RIC vol %for the same infill vol %. Atomostically distributed particles shouldresult in complete RIC vol % coverage. In FIG. 6, 610 shows fraction ofvolume affected to RIC generating particles in the host material asdetermined by running simulations of randomly generated distributedparticles of 5 micron diameter, 612 shows fraction of volume affected toRIC generating particles in the host material as determined by runningsimulations of randomly generated distributed particles of 10 microndiameter, 614 shows fraction of volume affected to RIC generatingparticles in the host material as determined by running simulations ofrandomly generated distributed particles of 15 micron diameter, 616shows fraction of volume affected to RIC generating particles in thehost material as determined by running simulations of randomly generateddistributed particles of 20 micron diameter, 618 shows fraction ofvolume affected to RIC generating particles in the host material asdetermined by running simulations of randomly generated distributedparticles of 25 micron diameter, and 620 shows fraction of volumeaffected to RIC generating particles in the host material as determinedby running simulations of randomly generated distributed particles of 30micron diameter.

FIG. 6 shows statistical distributions for varying particle sizes at 4vol % infill within a PbTe ATEG foot. As can be observed from this plotthe particle size effects the RIC vol % infill fraction, with smallerparticle sizes helping to distribute the RIC influenced areas throughoutthe material. As the particle size decreases, the RIC infill percentreaches its maximum value. With atomistic distribution, virtually theentire sample should be under RIC influence. This should be the case forthe various borides and boron compounds that have been identified aspotential ATEG materials.

The same simulation was performed on various thermoelectric materials tostudy their behavior with various vol % infills, alpha sources, andparticle sizes, which is visible in FIG. 7.

FIG. 7 illustrates chart 700 showing particle size, alpha source type,and ATEG foot type effect on RIC vol % infill fraction, according tovarious exemplary embodiments. Solid lines represent 10B(n,α) particles,dashed lines represent 241Am particles, and dotted lines represent 238Puparticles. FIG. 7 shows how the actual ATEG foot material can affect theRIC coverage compared to others, which may be a factor in the decisionto choose specific materials over one another to SPS with the alphaparticle sources. The plot of FIG. 7 shows that 238Pu and 241Am effectthe material in a similar fashion, with the difference being thespecific activity of each radioisotope. While this simulation wasconducted on TEG materials at 4% infill, higher infill percentages seemuch larger RIC vol % infill fractions, particularly with 10B(n,α). InFIG. 7, 702 shows the fraction of volume affected by RIC generatedparticles in a NiO host material as a function of RIC generatingparticle size, 704 shows the fraction of volume affected by RICgenerated particles in a SiGe host material as a function of RICgenerating particle size, 706 shows the fraction of volume affected byRIC generated particles in a PbTe host material as a function of RICgenerating particle size, and 708 shows the fraction of volume affectedby RIC generated particles in a Bi2Te3 host material as a function ofRIC generating particle size.

Determining the impact on the efficiency of the thermoelectric generatoruses the RIC vol % infills determined from these simulations. When theseparticles are introduced to the ATEG matrix, they affect the electricalconductivity, Seebeck coefficient, and thermal conductivity. Asdiscussed earlier, the electrical conductivity is affected the most byradiation, while the thermal conductivity and Seebeck coefficientincrease but maintain the same magnitude. The particles introduced havetheir own electrical conductivity and thermal conductivity which caninfluence the new composite properties in potentially negative ways.Boron for example has a thermal conductivity of 27 W/mK, which is anorder of magnitude higher than the ATEG materials shown in FIG. 7.

The addition of boron particles increases the overall thermalconductivity of the material which can negatively affect the performanceof the ATEG. Pu and Am do not suffer as much from this issue as theirthermal conductivities, while greater than most thermoelectricmaterials, are still the same order magnitude as the ATEG feet. Theeffects of adding the filler material were taken into consideration todetermine any negative effects.

FIG. 8 illustrates chart 800 showing predicted PbTe enhanced efficiencydue to alpha sources within its material matrix, according to anexemplary embodiment. Line 810 represents 238Pu, line 820 represents241Am, and line 830 represents 10B particles. In one example, plutoniumhas a higher activity level than the other sources therefore resultingin a greater change in conductivity in its influenced area.

The degree of conductivity changes is under investigation as significantinformation over a range of materials is not available. Becauseplutonium decays at a faster rate than americium, the plutonium makesmore alpha particles per second, makes more ionizations, and explainswhy the Pu line has higher efficiency as shown. However, an experimentat Oak Ridge National Laboratory (ONRL) observed an increase of 400× inthe electrical conductivity of alumina when exposed to ionizingradiation at temperatures within the operating range of exemplaryembodiments detailed herewithin. Another experiment on ceramic materialsobserved over 10,000 times increase in electrical conductivity in a UVgrade sapphire sample inside a reactor core (for additional information,seehttps://www.sciencedirect.com/science/article/abs/pii/S0022311598003006).In at least one exemplary embodiment detailed herein, boron nitrideshowed a 50× increase in electrical conductivity when subjected to aneutron flux in a TRIGA reactor. A conductivity multiplier factor can beapplied to the material's electrical conductivity to simulate theexpected change in properties during experimental determination ofadditional materials.

The efficiency and figure of merit of the ATEG is determined fromequations 2 and 3 with modifications to the thermal and electricalconductivities due to the particles' effects. These results are visiblein FIG. 8 for a lead telluride sample with the three alpha sourcesstudied. Efficiencies were calculated for a standard temperature at 600K for the hot side and 350 K for the cold side. As observed earliersmaller particles have a greater effect at increasing the electricalconductivity and therefore the efficiency. The activity of 238Pu isroughly five times greater than that of 241Am for an equivalent mass.This means that the material around the 238Pu shows a higher dose, whichis directly linked to the conductivity multiplier.

The activity of 10B is dependent on the neutron source flux, in thiscase the radioactivity matched that of 241Am. FIG. 8 shows thisrelationship with 238Pu having a greater efficiency due to its higherelectrical conductivity. In this case, a conductivity multiplier of 10was used to simulate the effects of RIC. FIG. 8 shows that plutonium hasa greater effect on increasing efficiency because of its increasedreactivity, while the other two alpha sources show lower activitylevels. Because of the lower RIC vol % infill, 10_(B)(n,α) did not matchthat of 241Am, which had a higher vol % coverage.

FIG. 9 illustrates chart 900 showing figures of merit associated withthe ATEG created from PbTE materials with the alpha source particles,according to an exemplary embodiment. Reaching higher levels ofefficiency requires larger figures of merit. FIG. 10 illustrates chart1000 showing increases in efficiency and figure of merit occur atrelatively small changes in conductivity and shows expected PbTe TEGperformance with magnitude cond. change. FIG. 9 shows the associatedfigures of merit for the ATEG couple showing that significantly largerfigures of merits can be reached than previously possible throughconventional TEG technologies, even though the conductivity multiplierused to simulate this ATEG was significantly lower than that of previousstudies on other materials. Where the ZT references above for mostmaterials show a ZT around 1 as the common standard, while the chartshows Pu doped ATEGs reaching ZT values of 14. As shown in FIG. 9, line910 represents 238Pu, line 920 represents 241 Am, and 930 represents 10Bparticles. The ZT value is plotted in FIG. 9 versus the size ofparticles of those materials added to the TEG foot as a dopant. However,it has been observed that changes in conductivity on the level observedby ORNL are not necessary to raise the efficiency of the thermoelectricmaterial above 20-25%. This is visible in FIG. 10 where the conductivitymultiplier has been plotted against the change in figure of merit andthe efficiency of a PbTe—PbTe ATEG. As shown in FIG. 10, changes of25-50 times the normal conductivity can yield drastic changes inefficiency and figure of merit as shown along lines 1010 and 1020. For a50× increase, the efficiency is near 33% and the ZT is near 30. Line1020 corresponds to the ATEG efficiency as a function of theconductivity multiplier. At changes in conductivity seen with ORNL, theefficiency would increase to 40.8%, which is roughly 98% that of Carnotefficiency.

This multiplication factor would coincide with a large dose ofradiation. Radioisotope particles may have a continued dose rate thatslowly decays over time in conjunction with their half-life. Meaningthat they may never exceed the expected efficiency based on theirconductivity multiplier and their RIC coverage area. With a (n,α)particle such as 10B, the dose rate can be increased and decreaseddepending on the desired efficiency. This is because the alpha particledose is dependent on the neutron flux, which can be adjusted inside ofthe reactor. Simply increasing and decreasing the neutron flux maychange the electrical conductivity of the ATEG which would not bepossible with a constant radiation source such as radioisotopes.

FIGS. 11A-11B illustrate charts 1100 and 1102 showing the performance ofa PbTe ATEG under various temperature gradients. Variation in plot isdue to temperature dependent material properties of PbTe. Chart 1100shows PbTe ATEG with ˜30% efficiency with mission temperature gradient.Chart 1100 shows the efficiency of a PbTe based ATEG as a function ofhot side temperature with a cold side temperature of 325K at 1110, of350K at 1112, of 375K at 1114, and of 400K at 1116. Chart 1102 shows thefigure of merit of PbTe ATEG exceeding currently available ZT value.Chart 1102 shows figure of merit (ZT) of a PbTe based ATEG as a functionof hot side temperature with a cold side temperature of 325K at 1120, of350K at 1122, of 375K at 1124, and of 400K at 1126.

Utilizing the simulations and models derived above, a comprehensive ATEGtool was developed to determine performance characteristics and physicalproperties of the ATEGs. This tool has shown that these ATEGs have thecapability of reaching extremely high efficiencies. Models have exceededthe expectation of reaching efficiency values of 20% with all threealpha sources. As shown in FIG. 11, one such ATEG with a PbTe as thep-type and n-type foot with a 10B(n,α) source with a conductivitymultiplication factor of 50 and a 7% volume infill. At the temperaturegradient for expected missions, an expected efficiency of 30% should bereachable for this ATEG. Variation in the figure of merit plot is due tothe thermoelectric property dependence on temperature in PbTe. Theseresults will enable space missions including future deep spaceexploration missions, that require high power density sources with nomoving parts. The power derived from the ATEG can be determined from thefollowing equations. The Seebeck coefficient is calculated according toEquation 4:

α=α_(p)−α_(n)  (4)

wherein p and n denotes the p-type and n-type material and a representsthe Seebeck coefficient. The total resistance of the thermoelectriccouple is calculated according to Equation 5:

$\begin{matrix}{R = {\frac{\rho_{p}L_{p}}{A_{p}} + \frac{\rho_{n}l_{n}}{A_{n}}}} & (5)\end{matrix}$

wherein ρ is the materials electrical resistance (note conductance isthe reciprocal of this), L is the length of thermoelectric material, andA is the cross sectional area of the thermoelectric foot. The thermalconductivity of couple is calculated according to Equation 6:

$\begin{matrix}{\kappa = {\frac{\kappa_{p}A_{p}}{L_{p}} + \frac{\kappa_{n}A_{n}}{L_{n}}}} & (6)\end{matrix}$

wherein K is the thermal conductivity of the specific material. Voltageproduced by the ATEG is a function of the Seebeck coefficient, ATEGresistance, load resistance and the number of couples in the ATEG.Current is dependent on the same parameters as the voltage as calculatedaccording to Equations 7 and 8:

$\begin{matrix}{V = {\frac{n\;{\alpha\left( {T_{h} - T_{c}} \right)}}{\frac{R_{L}}{R} + 1}\left( \frac{R_{L}}{R} \right)}} & (7) \\{I = \frac{\alpha\left( {T_{h} - T_{c}} \right)}{R_{L} + R}} & (8)\end{matrix}$

wherein RL denotes the load resistance; maximum power is reached whenthe load resistance matches the ATEG internal resistance. Current,voltage and power are calculated according to Equations 9-11:

$\begin{matrix}{V = \frac{n\;{\alpha\left( {T_{h} - T_{c}} \right)}}{2}} & (9) \\{I = \frac{\alpha\left( {T_{h} - T_{c}} \right)}{2R}} & (10) \\{P = \frac{n{\alpha^{2}\left( {T_{h} - T_{c}} \right)}^{2}}{4R}} & (11)\end{matrix}$

As shown in Equation 11, to increase power, the resistance of the systemmust be lowered, while Seebeck coefficient and temperature gradient mustbe increased. These equations can be used to determine several keyparameters of the ATEG power system.

FIGS. 12A-12F illustrate charts 1202-1212 showing performancecharacteristics of an ATEG system that produces 3 kW of electricalpower, noting that masses are of the ATEG only and do not representoverall power production system. ATEG feet used at n-type and p-typePbTe with 7 vol % infill and a 40 times conductivity multiplier. Chart1202 shows the number of thermoelectric couples used to provide 3 kw ofelectric power as a function of hot side temperature. Four cold sidetemperatures were used to form the four different lines, with a coldside temperature of 325K at 1220, of 350K at 1222, of 375K at 1224, of400K at 1226. For low temperature differences, more couples are needed.Chart 1204 shows the total current flowing through the ATEG as afunction of hot side temperature for four different cold sidetemperatures, with a cold side temperature of 325K at 1230, of 350K at1232, of 375K at 1234, of 400K at 1236. Chart 1206 shows specific power(i.e. how many watts of electrical power can be produced per kg of ATEGconverter) of the system as a function of hot side temperatures for fourdifferent cold side temperatures, with a cold side temperature of 325Kat 1240, of 350K at 1242, of 375K at 1244, of 400K at 1246. Chart 1208shows power density (i.e. how many watts of electrical power can beproduced per square centimeter of ATEGs laid out over the heat source)of the system as a function of hot side temperature for four differentcold side temperatures, with a cold side temperature of 325K at 1250, of350K at 1252, of 375K at 1254, of 400K at 1256. Chart 1210 shows powergenerated in each thermoelectric couple in the array as a function ofhot side temperature for four different cold side temperatures, with acold side temperature of 325K at 1260, of 350K at 1262, of 375K at 1264,of 400K at 1266. Chart 1212 shows total mass of the thermocouple feet,not counting mounting, housing, etc. necessary to generate 3 kW as afunction of hot side temperature for four different cold sidetemperatures, with a cold side temperature of 325K at 1270, of 350K at1272, of 375K at 1274, of 400K at 1276

In charts 1202-1212, the mass of the ATEG is determined through thephysical properties of the ATEG feet materials and current ATEGtechnologies (primarily for the insulators, conducive connectors, andsolder). This results in a high specific power as only the weight of theATEGs are taken into consideration with this calculation. The ATEGconversion system mass is miniscule compared to the reactor mass and theATEG collects a large amount of power for a given surface area.Calculations for the figure above only take into account ATEG couplesconnected in series. A more realistic system would see ATEGs connectedin a system of series and parallel connections to introduce someredundancy within the system in case of failures in the ATEG couplerconnections. This arrangement would increase system mass slightly, butdoes not appear to be a major system driver. In some of the ATEGmaterials, the addition of the RIC filler actually decreases the mass ofthe system slightly, specifically with 10B which is much lighter thanmost of its ATEG matrices.

These systems were also validated in the computer program COMSOL to showhow an increase in conductivity may affect an assembled ATEG. Thematerial conductivity values were changed in a similar manner to showthe RIC particles would affect the thermoelectric generator.

FIGS. 13A-F illustrate COMSOL simulations 1302-1312 of a bismuthtelluride (Bi2Te3) p-type and n-type ATEG with applied conductivitychanges ranging from 5 to 100. Large increases in figure of merit andthe efficiency can be seen in the figure with the figure of merit (ZT)being shown on the top three images of FIGS. 13A-C. Figure of merit fora 5 times conductivity multiplier of the electrical conductivity of thematerial is shown at 1302 with scale 1320 a showing values for variousareas and showing maximum values of change of 7 in the image. Figure ofmerit for a 20 times conductivity multiplier of the electricalconductivity of the material is shown at 1304 with scale 1320 b showingvalues for various areas and showing maximum values of change of 28 inthe image. Figure of merit for a 100 times conductivity multiplier ofthe electrical conductivity of the material is shown at 1306 with scale1320 c showing values for various areas and showing maximum values ofchange of 140 in the image. The maximum values change from 7 to 28 to140 in the three images. Thus, the ZT value for each multiplierincreases well beyond the ZT values of any other material. The bottomthree images of FIGS. 13D-F shown at 1308, 1310, and 1312 have the sameelectrical conductivity multipliers as above, but instead of modelingthe ZT value efficiency (measured as the ratio of electrical powergenerated over the thermal power passing through the foot) is listed.Efficiency for a 5 times conductivity multiplier of the electricalconductivity of the material is shown at 1308 with the scale to theright (at 1320 d) showing the maximum efficiency value reaching 0.22.Efficiency for a 20 times conductivity multiplier of the electricalconductivity of the material is shown at 1310 with the scale to theright at 1320 e showing the maximum efficiency values reaching 0.3.Efficiency for a 100 times conductivity multiplier of the electricalconductivity of the material is shown at 1312 with the scale to theright at 1320 f showing the maximum efficiency values reaching 0.36.These values of maximum efficiency are well beyond traditional TEGvalues. Original figures of merit and efficiency were between 0.8 and1.4 and 8-11%. Here, the figure of merit and the efficiency changed asthe degree of conductivity change was increased. It should be noted thatthe original figure of merit range was between 0.8 and 1.4 for this ATEGcombination of bismuth telluride p-type and n-type while the efficiencyvaried between 8-11%. Even with a conductivity increase of only 5×, thefigure of merit and efficiency increased to between 4-7 and 18-22%.Larger increases in efficiency and figure of merit are seen with modestincreases well below literature values, including values reported fromother studies or existing units. NASA MMRTG has an efficiency of 5-7%.

Experimental Results

Experiments were conducted to validate radiation induced conductivity inpotential thermoelectric materials. While the most heavily documentedRIC cases have involved aluminum oxide and other insulators, none ofthem contained 10B for an alpha source. Boron nitride (BN) was chosen asthe material to be tested for its potential to demonstrate the keyconcept of the (n,α) source, and that it is considered a wide band gapinsulator. Within the BN, 20% of the boron content would comprise 10Bwhich should have a uniform distribution throughout the compound. ATRIGA Mark II Nuclear Research Reactor at Kansas State University (KSU)was used as a neutron source to generate the alpha particles. Leads wereattached to the BN sample to take insitu conductivity measurements whilethe reactor was being operated. Contained within a polyethylenecontainer, the BN sample was subjected to the neutron flux at variouspower levels to determine its resistive response.

FIG. 14 illustrates chart 1400 showing Boron Nitride resistance effectsdue to neutron flux from nuclear reactor. Clear dependence on neutronflux is shown as resistance decreases with increases in power. This isevidenced, for example, as the neutron flux in the reactor increasesproportionately to the power of the reactor. As the reactor powerincreases, so does the neutron flux to which the sample is exposed. Thechart shows the electrical conductivity decreasing every time the powerof the reactor is increased. FIG. 14 shows the power between OW to 20 kWof thermal power. An insulation resistance meter was utilized todetermine the resistance of the material. The initial resistance of thesample was just over 4 GΩ and slowly rose to above 5 GΩ before the RICeffects were visible. This effect is visible in FIG. 14 with powerintervals marked between dashed lines. Insulators typically exhibit anincrease in resistivity as they are measured from capacitive effectswhich was observed during this experiment and previous experimentsobserved without a neutron flux. An initial decrease in resistance isobserved as the reactor is stepped up to 40 Watts. As the reactor powerincreases to 160 Watts, a more noticeable drop in resistance is visible.This trend continues as the power/neutron flux increases. Larger jumpsin resistance appear to be more prevalent at lower increases in power.As shown in FIG. 14, 1402 shows resistance values of boron nitride as afunction of time during a test that gradually increased reactor powerand dose to the material and 1404 shows reactor power as a function oftime.

FIG. 15 illustrates chart 1500 showing Boron Nitride complete test up to½ Mega-Watt (MW) power level observing ˜50 times conductivitymultiplier. Previous data shows that a 50 times conductivity multiplieris more than suitable to significantly change the performance of anATEG. Boron Nitride was tested up to ½ MW to determine the ATEG'sresistance response at this power/flux level. As shown in FIG. 15, at1502 resistance values of boron nitride as a function of time during atest that gradually increased reactor power and dose to the material andat 1504 reactor power as a function of time. As shown in FIG. 15, theentire test was conducted over the course of one hour. After the reactorreached a level of 20 kW and was held for a short period of time, acontrol rod was removed as the reactor was ramped up to 40 kW. Thiscaused a decrease in reactor power which also resulted in an increase inconductivity. This again shows an extremely responsive relationship tothe reactors power level/neutron flux. Power levels were increased againto 80 kW, 160 kW, 300 kW and finally 540 kW. During this time theresistance meter stopped recording but continued to display resistance.Personnel, except the reactor supervisor, were stationed inside of thecontrol room and away from the meter and were unable to manually restartit. However, the reactor supervisor relayed data points at 300 kW and540 kW, which were 183 MOhm and 140 MOhm respectively. These points havebeen marked on FIG. 15 to show their respective position at that powerlevel. It is clear from these plots that the increase in conductivity isnot linear with the increase in power. At smaller activity levels withinthe reactor, there were larger gains in conductivity observed. Thisshows that large changes in conductivity are likely especially atpower/flux levels that the SPEAR reactor may produce. While BN is nottypically used as a thermoelectric material, there have been severalstudies that show its potential.

Although the 400-10,000× increase in electrical conductivity was notobserved by the BN in this experiment, there was at least a ˜50×increase in electrical conductivity observed. Referencing FIG. 15, thisslight increase in conductivity could be enough to reach efficiencyvalues greater than 20% depending on other material properties.

FIGS. 16A through 16I illustrate COMSOL modeling results 1602-1612 forBN thermoelectric junction for conductivity multipliers of 1, 30, and50×. Current density, power levels, and ZT values all improvedrastically with the increased conductivity values measured at the KSUreactor. With a highly resistive material, the efficiency of thethermoelectric generator is small. However, there should be a slight butnoticeable change in voltage/power/current from the ATEG couple. WhileSeebeck coefficients for most thermoelectric and semiconductors areknown, Seebeck coefficients for insulators are less studied, thereforeSeebeck coefficients for BN were estimated from literature values.COMSOL was used to simulate an ATEG couple that could produce power onthe micro-Watt scale based on known and estimated thermoelectricproperties of BN. The results of this brief study are visible in FIGS.16A through 16I. Voltage is not shown, as this is dependent on theSeebeck coefficient and maintained a consistent 0.4V drop. Allsimulations were conducted with a 600K hot side and a 350 K cold side.From this model, it can be seen that there are noticeable changes tocurrent, power levels, and ZT values with increases in conductivity. BN,pending further investigation into Seebeck coefficient, is consideredthe p-type foot, while the ntype foot was modeled as SiGe. As shown inFIGS. 16A through 16I, the top, middle, and bottom rows are measuringthe electrical current, power generated, and ZT values, respectively.The left, middle, and right columns change the electrical conductivityby a factor of 1, 30, and 50 times the original value. The material inquestion is boron nitride. The simulation shows that the current, power,and ZT all increase greatly with the changes measured from theexperiment. As shown in FIGS. 16A through 16I, 1614 indicates ZT valuesof the junction for a conductivity multiplier of 1, 1616 indicates ZTvalues of the junction for a conductivity multiplier of 30, and 1618indicates ZT values of the junction for a conductivity multiplier of 50.

ATEG Materials

Many of the thermoelectric materials described in the sections abovemust be created via spark plasma sintering (SPS) if they are to becreated with the radioisotope or the (n,α) if not already embeddedwithin the material. However, several thermoelectric materials with10B(n,α) have already been studied and can be obtained for futureexperiments. A few thermoelectric materials containing BN in the form ofquantum dots, thin films, and nano-ribbons/tubes have been studied. Thisis advantageous as our BN based system takes bulk BN material andincreases its electrical conductivity significantly.

FIG. 17 illustrates chart 1700 showing Seebeck coefficient of boronbased thermoelectric materials shown as sample numbers 1702, 1704, 1706,1708, 1710 1712, 1714. These materials are predicted to behave exemplarywhen augmented with RIC. FIG. 18 illustrates chart 1800 showing responseof electrical conductivity of showing boron based thermoelectricmaterials shown as sample number 1802, 1804, 1806, 1808, 1810, 1812,1814. AZT of 2.5 is possible with graphene/h-BN (hexagonal BN)superlattice monolayers. If the changes in conductivity observed fromthe KSU experiment hold true for BN to this superlattice, a ZT potentialof greater than 100 is theoretically possible which corresponds to anefficiency of 35% with SPEAR's temperature gradient. This would farexceed any thermoelectric technologies currently available. In anotherstudy conducted by Algharagholy et al. involving graphene-boron nitridehetero-structures of various widths a ZT value of 0.9 was theoreticallypossible. Again, if the same change in conductivity is observed as theBN experiment, this would result in a ZT value of >40 corresponding to a32% efficiency. It would appear that despite BN's poor bulkthermoelectric properties, their use inside the technologies above canincrease ZT values to usable levels, and potential RIC behavior crossover to these materials can accelerate these values beyond what waspreviously thought possible. The current record ZT value of 7.4 wasmeasured in hybrid MoS2/MoSe2 nanoribbons at 800K by Ouyang et al. Thedisclosed ATEG technologies and projections with BN alone wouldsignificantly pass this record.

Other materials containing boron have been extensively studied for theirpotential use in thermoelectric generators. Many of them are on the cuspof high ZT values, most being held back by lower than average electricalconductivities for thermoelectric materials. One such material, AlMgB14has been shown to have high Seebeck coefficients and moderate electricalconductivities based on its mixture ratios. Miura et al. has shown thatAlMgB14 can reach Seebeck coefficients in upwards of 250-450 μV/K whichis competitive with most thermoelectric materials. Through slightchanges in material composition during the SPS process, Miura et al. wasable to create an AlMgB14 sample with a −500 μV/K Seebeckcoefficient—meaning that an ATEG could be created with virtually thesame material, limiting issues with thermal expansion coefficients andother stresses that the ATEG might endure with varying ATEG feet. Thevarying Seebeck Coefficient and electrical conductivity is visible inFIGS. 17-18 (for additional information, see S. Miura, H. Sasaki, K.-i.Takagi and T. Fujima, “Effect of varying mixture ratio of raw materialpowders on the thermoelectric properties of AlMgB14-based materialsprepared by spark plasma sintering,” Journal of Physics and Chemistry inSolids, vol. 75, no. 8, 2014.). The Seebeck coefficients are very highfor alumina, however, the electrical conductivity is much lower comparedto traditional thermoelectric materials such as PbTe and SiGe. While theelectrical conductivity restricts the efficiency of alumina, it has beenshown that slight changes in its composition can have drastic changes inthese levels. Combined with RIC enhancing effects, this could result ina much higher efficiency and ZT value. AlMgB14 is also advantageous inthat the materials are not considered rare earth borides (REB) makingproduction cheaper and more sustainable on a larger scale.

While AlMgB14 has a great potential for thermoelectrics at a sustainableand economical price point, there are several other REBs that sit on thecusp of advancing thermoelectric technologies. Materials identified withthe greatest potential for ATEG feet include ErB44Si2, YB66, YB44Si2,SmB60, SmB62, TbB44Si2, and ErB66. The thermoelectric properties ofthese materials can be found in TABLE 2. While these materials are notcurrently considered high performing TEGs, with modification to theirelectrical properties via RIC they can reach high ZT values making thempromising candidates for ATEGs. Embodiments further contemplate lithium-or sulfur-containing materials which absorb neutrons and emit ionizingradiation. Fission products or other elements which emit alpha, beta,gamma, neutron, or other ionizing radiation may also be used to inducethe RIC.

TABLE 2 Seebeck Thermal Electrical Material Coefficient ConductivityResistance ErB₄₄Si₂ 50-220 [37] 1.6-2.7 [37] 0.0087-135 [38] YB₆₆205-752 [39] 2.49-3.74 [39] 0.0035-157 [39] YB₄₄Si₂ 80-200 [38] ~2.7[38] 9e−4-0.02 [38] SmB₆₀ 197-567 [39] 2.53-3.17 [39] 1.33e−4-0.332 [39]SmB₆₂ 207-557 [39] 2.06-2.76 [39] 1.54e−4-0.625 [39] TbB₄₄Si₂ 70-140[38] ~2.7 [38] 9e−4-0.0045 [38] ErB₆₆ 200-700 [38] ~2.7 [38] 0.0085-207[38]

TABLE 2 shows material properties for various rare earth borides thatcan potentially be used as advanced thermoelectric generators, withSeebeck coefficient in V/k, Thermal conductivity in w/m*k, electricalresistance in Ohm*m. These materials are mostly held back due to theirhigher electrical resistivity compared to traditional TEG materials. Itis predicted that they may behave more like an insulator thantraditional TEG materials so changes in conductivity should be morenoticeable. Small changes in conductivity are enough to achievenoticeably large increases in efficiency. As shown in TABLE 2, bracketedreference [37] is T. Mori, “High Temperature Boron-based ThermoelectricMaterials,” Material Matters, vol. 4, no. 2, 2011, bracketed reference[38] is T. Mori, “High temperature thermoelectric properties of B12icosahedral cluster-containing rare earth boride crystals,” Journal ofApplied Physics, 2005, and bracketed reference [39] is A. Sussardi et.al, “Enhanced thermoelectric properties of samarium boride,” Journal ofMateriomics, vol. 1, pp. 196-204, 2015.

FIGS. 19A-C illustrate charts 1902, 1904, 1906 showing computationalmodeling results for a rare earth boron based ATEGs at different levelsof RIC. Chart 1902 shows conversion efficiency of an unmodified SmB60ATEG as a function of hot side temperature with a cold side temperatureof 325K at 1910, of 350K at 1912, of 375K at 1914, of 400K at 1916.Chart 1904 shows conversion efficiency of a lightly doped SmB60 ATEG asa function of hot side temperature with a cold side temperature of 325Kat 1920, of 350K at 1922, of 375K at 1924, of 400K at 1926. Chart 1906shows conversion efficiency of a heavily doped SmB60 ATEG as a functionof hot side temperature with a cold side temperature of 325K at 1930, of350K at 1932, of 375K at 1934, of 400K at 1936. Chart 1906 shows RICmultiplier equivalent to 400 times applied to the material. Efficienciesfar exceed traditional TEGs. These materials, if shown that they cannotreach the required conductivity levels to increase the ZT to therequired levels for SPEAR, can still demonstrate the RICs ability toincrease ZT substantially with the change in conductivity observed bythe BN. If the conductivity change matches closer to the aluminasamples, the conductivity should reach into efficiencies greater than20%. As shown in FIGS. 19A-C, a simulation conducted with SmB60 withvarious levels of conductivity applied. As shown, the efficiency withoutRIC effects applied is lower than current TEG technologies. With thesame change in conductivity applied to the ATEG as was observed in theKSU experiment, the efficiency increases to levels matching currenttechnology capabilities. SmB60, from Table 2 above, as well as otherREB, excel at high temperatures, which, as a result of largertemperature gradients, would increase the expected conductivity evenfurther. The higher electrical resistance of these materials brings themcloser to being insulators than the traditional thermoelectricmaterials. This fact may lead them to behave in a similar fashion to thealumina and BN, where RIC was observed in these insulators.

The technology behind ATEGs shows great promise in expanding upon theefficiency of current TEG technologies. This enables the SPEAR missionto maintain its very low mass and propel itself with NEP to Europa.Improving the amount of power extracted via solid state energy methodshas untold scientific benefits for deep space missions.

ATEG Conversion System Conclusions

The effects of radiation induced conductivity were shown experimentallyin a neutron field at KSU, and the electrical conductivity of the boronnitride did indeed change by up to 50×. Embodiments contemplate BN thinfilm thermoelectrics as viable ATEGs. Other boron based TEGs are likelyto be greatly enhanced as well, as their electrical conductivity isoften their weakness and the RIC may improve this.

SPEAR faces very unique radiation environments from its own power sourceas well as the different environments it may encounter. The reactor hasbeen designed in such a way to maintain criticality at moderatetemperatures. These temperatures are comparable to RTGs and contributeto its successful design as a lightweight reactor that can produce asignificant amount of power due to the ATEG power conversion system.

SPEAR utilizes its geometry and radiation shield to protect the valuablepayload from the reactor during operation. This was modeled to determinethe expected dose rate for the CubeSats. SPEAR may also be subject tovarious radiation environments throughout its journey to Europa. Themost prominent being the Van Allen Radiation belts as well as thepowerful Jovian radiation fields. These radiation environments werestudied to determine how they may affect the spacecraft as well as theCubeSats once they are deployed in Europa orbit.

Reactor Design

The SPEAR nano-reactor produces 15 kW of thermal power and utilizing theATEG power conversion system is estimated to produce 3 kW of electricalpower with a minimum conversion efficiency of 20%. This is aconservative estimate and our studies have shown the potential for up to30% efficiency for 4500 kW of electrical power. The unique choice ofmaterials and geometry of the reactor make it uniquely suited to producethis power in a relatively small form factor with low enriched uranium.

The underlying concept behind the reactor design is to decrease the massby using the high efficiency ATEGs, new materials, and a custom design.With the increased efficiency of the ATEGs, the SPEAR reactor does notrequire extremely high operating temperatures that often accompanydynamic cycles. The conversion system was designed to be on the sametemperature scale as an MMRTG, which also uses TEG conversion, and thehot and cold sides are 600K and 350K, respectively. This allows forlithium hydride to be used as a moderator material instead of zirconiumcarbide. LiH is a very effective moderator due to its low Z numberconstituents but is usually not considered for nuclear applications dueto the low melting temperature. The SPEAR is also able to use uraniummetal instead of a carbide or oxide form, which keeps design simple andreduces unnecessary mass. By taking these steps to shrink the reactormass down as far as possible, the entire ship, propellant, and payloadmasses can be reduced to create small and affordable spacecraft.

Geometry Selection

The scale of reactor components was determined through trials in MonteCarlo Neutral Particle code, version six (MCNP6). The design goal was tocreate the smallest possible reactor to reach a criticality greater thanone. Other considerations include heat pipe spacing, mass efficiency,and manufacturability. Trials in MCNP6 were most successful the closerthe overall dimensions were to equal. This is to say the ratio ofreactor criticality per unit volume tended to be higher when the base,width, and height were similar. The final iteration of the geometryproduced the following results in MCNP6.

TABLE 3 Number of Total Initial Control Rod Steady State Standard Numberof Cycles State Criticality Deviation Cycles Discarded No Insertion1.01437 0.00012 500 100 Fully Inserted 0.98793 0.00011 500 100

TABLE 3 shows MCNP6 Results for final geometry sizing.

FIG. 20A illustrates SPEAR reactor 2000, according to an exemplaryembodiment. SPEAR reactor comprises heat pipes, moderators, reflectors,control rod, and low enriched uranium fuel. The hot side of the ATEGsare connected to the mercury heat plate while the cold side areconnected to the heat rejection system connected to the radiators alongthe side of the spacecraft. FIG. 20A also shows vacuum 2000, LiHmoderator 2010, uranium fuel 2012, beryllium reflector 2014, boroncarbide control rod 2016, and heat pipes 2018. Note that these elementscorrespond to the reactor cutaway shown in FIG. 1.

FIG. 20B illustrates layout of SPEAR reactor, according to an exemplaryembodiment. MCNP (Monte Carlo Neutral Particle transport code) model wasused to model SPEAR reactor. LEU (Low Enriched Uranium) and LiH (LithiumHydride) moderator are organized into concentric rings with equallyspaced heat pipes embedded into the LEU. Boron control rod is located atcenter. Beryllium reflector encases LiH and LEU layers. The reactor coremay be composed of reactor materials, such as low-enriched uranium,lithium hydride, beryllium, and boron. Everything may be contained usingaluminum for its high strength to weight and resistance to neutrondamage. The heat pipes may use mercury as the working fluid, matched tothe planned operational temperature of the reactor of 600K.

The reactor was organized cylindrically to promote radial symmetry ofthe layers of moderator, fissionable material, and heat pipes shown inFIGS. 20A-20B. This geometry of fuel/heat pipes helps reduce thepresence of heat concentrations throughout the reactor. This alsopermits a single control rod to bring the system criticality below onewhen placed in the very center, the point of maximum neutron flux. Asingle control rod recues complexities involved with controlling thereactor and reduces failure modes that a multi-control rod system maysee. This reduction of failure modes is paramount in such a mission witha relatively long operating period. Prior to activation of the reactorin LEO, the control rod may be fully inserted to prevent fission fromoccurring.

The limiting factor in this design is the maximum temperature reached bythe uranium fuel during operation. If the fuel reaches high enoughtemperatures, the hot fuel in contact with the LiH moderator will causethe moderator to dissociate and may dissociate the LiH moderator andnegatively affect performance. Uranium metal has a rather poor thermalconductivity, and heat pipes were required to be placed close togetherto avoid going over the maximum allowable temperature. Doping the fuelrings with a small amount of alloying material may allow for increasedthermal conductivity, and thus fewer heat pipes. Geometric optimizationof the core could also assist in this aspect, as rings are neutronicallyfunctional but potentially not ideal for heat transport.

A mass budget estimate was generated according to the materialrequirements and sizing of the MCNP model and is visible in TABLE 4.Assumptions disregard the negligible mass contribution of the internalthin walls for the physical separation of the reactor componentmaterials. The mercury heat pipes contain a mixture of vapor and liquid,but for simplification were approximated to be full density. The heatpipe walls, and internal structure were also approximated to be mercury.As mercury is very dense compared to structural metals, theseapproximations are both overestimations.

TABLE 4 Outer Inner Diameter Diameter Length Volume Density MassComponent Quantity (cm) (cm) (cm) (cm³) (g/cm³) (kg) Beryllium 1 40 2820 12817.70 1.85 23.71 Lithium Hydride 1 5 1 20 376.99 0.82 0.31 Ring 1Lithium Hydride 1 14 8 20 2073.45 0.82 1.70 Ring 2 Lithium Hydride 123.25 17 20 3951.53 0.82 3.24 Ring 3 Lithium Hydride 1 28 25.75 201899.68 0.82 1.56 Ring 4 Uranium 1 8 5 20 612.61 19.1 11.70 Ring 1Uranium 1 17 14 20 1460.84 19.1 27.90 Ring 2 Uranium 1 25.75 23.25 201924.23 19.1 36.75 Ring 3 Mercury Heat 22 1 0 32 25.13 13.56 7.50 PipesBeryllium 2 40 0 6 7539.82 1.85 27.90 Caps Boron Control 1 2 0 20 62.832.37 0.15 Rod Total Mass 142.42 (kg) Total LEU 76.36 Mass (kg)

TABLE 4 shows the mass budget estimate broken down by component.

Advantages of LEU

Low-Enriched Uranium (LEU) comprises of <20% U-235, with the remainderbeing U-238. Highly Enriched Uranium (HEU) contains more 235U whichallows for a greater fraction of neutrons to cause fission allowing fora smaller volume/mass of uranium to reach criticality. While using HEUmay produce a smaller reactor, the proliferation of HEU is a primaryconcern for the US government and other national governments making theuse of HEU a significantly more difficult for private companies to workwith. There are greater security, transportation, and launchrequirements with HEU reactors and material which increases programcosts by millions of dollars.

Although HEU reactors may reduce launch costs because of lowered reactormass, it is likely that a specialized and larger standby force would beneeded for an HEU launch for recovery efforts to prevent HEU fromfalling into the wrong hands in case of an accident. While thegovernment may have the ability to launch HEU reactors at lower costs,commercial entities may most likely by restricted to only LEU as it maybe much cheaper. By some estimates, the cost reduction of commercialspace companies developing a reactor would be 10% to 50% that of onedeveloped by the government. The uranium within the SPEAR nano-reactormay be enriched to 19.75% to remain classified as LEU avoiding securityand regulation issues. Producing only 15 kWt of power also reduces thesize of the reactor as most NEP reactors are designed in the MWt powerrange which would increase launch costs and reactor complexity.

Advantages of LiH

When combined with an appropriate moderator and encased in a reflector,LEU is perfectly suitable for energy generation on scale suitable for aspacecraft the size of SPEAR with limited mass penalties. Moderatorsslow down neutrons emitted by the uranium, promoting additional fission.Reflector material helps keep more of the neutron material travellingthrough fissionable material. These two in combination allow a reactorbuilt with LEU to reach criticality well below LEU's critical mass.

LiH was selected as the moderator due to its high hydrogen density.Hydrogen atoms are the most efficient particles for slowing down fastneutrons since both have the same mass. Other materials, such asmethane, offer similar performance but are not solid at the systemoperating temperatures, making them harder to implement and handle.Graphite, typically used as a moderator for its high operatingtemperature, lacks large amounts of hydrogen. Utilizing LiH increasesthe amount of hydrogen but reduces the operating temperature from 4000Kfor graphite down to 961K for LiH. This reduction in operatingtemperature is compensated for the large increases in efficiency seenwith the ATEG conversion system. LiH also has the added benefit ofreducing the mass of the core as LiH is less dense than graphite.

Heat pipes may be located directly inside of the uranium fuel as that iswhere the heat is generated and may have the highest temperatures.Removing the heat at the source may assist in reducing temperaturepeaking and limit the effects of the poor thermal conductivity of themoderator.

ATEG Location

The ATEGs are located at the base of the reactor assembly. The verticalorientation of the heat pipes allows for a single heat diffuser toaccept input from all the heat pipes to the top of the ATEGs. The coldside of the ATEGs may connect to another heat diffuser plate with aworking fluid that transfers heat to the radiators to dissipate theheat. This radiator working fluid in conjunction with the radiators maymaintain the cold side temperature of the ATEGs at 350K.

As discussed in the previous sections, ATEGs utilizing (n,α)interactions require a thermal neutron source to achieve their expectedefficiencies. Therefore, the ATEGs have been placed as close as possibleto the reactor to maximize their exposure to the neutrons coming fromthe reactor. Increasing the efficiencies of these ATEGs would requireincreasing the flux rate from the reactor as this would increase boththe ionizations observed in the ATEGs as well as increase thetemperature of the reactor. ATEGs with radioisotope sources would notnecessarily require close proximity to the reactor, but the reduction inheat pipe mass is an important factor in maintaining a lightweightspacecraft.

Neutron Flux from Reactor

Analyzing the neutron flux is important both for determining theefficiency of the ATEGs and for shielding design. The neutron and photonflux were determined using MCNP6. Measurements were taken at 25 cm and350 cm from the center of the reactor. An averaged surface flux tallymeasured the average neutron flux through a sphere with 25 cm radius,placing the location where the surface flux tally was measured justoutside the reactor surface. This value of the neutron flux is importantas ATEG performance is directly dependent on the neutron flux when usinga (n,α) ATEG. Another F2 averaged surface flux tally was taken using twocircular surfaces whose area was equal to the planned shieldinggeometry.

Shield Design

SPEAR's shadow shield has been designed to protect the vital areas ofthe spacecraft from the harmful radiation that the reactor produces. Theshield takes the shape of a cross to match the orientation of theCubeSat bays. This geometric configuration of the shield conforms to howthe CubeSats are currently orientated for deployment at Europa. Theneutron shield itself is made from tetramethylammonium borohydride (TMABC4H16NB), which is a relatively light weight material that is useful asa shielding material in nuclear system. The hydrogen density within TMABallows it to thermalize neutrons and the boron, with a naturally largeneutron capture cross section, absorbs neutrons coming from the reactor.As disclosed herein, the neutron flux is reduced by four orders ofmagnitude.

FIGS. 21-23 illustrate portable SPEAR power source, according to anexemplary embodiment, with 2100 indicating external housing, 2102indicating SPEAR reactor cores, 2104 indicating radiator fins for wasteheat rejection, 2106 indicating radiation shielding, and 2108 indicatingfans for cooling radiator fins. FIG. 24 illustrates SPEAR spacecraftwith CubeSat swarm payload. The advanced thermoelectric generatorsenable massive efficiency increases over traditional thermoelectricgenerators reaching efficiencies matching or exceeding current sterlingor dynamic power systems. A lightweight and compact form factor enableSPEAR to be launched on small/medium launch vehicles allowing for muchlower launch costs. SPEAR has the potential to revolutionize theaccessibility to deep space for both government use and privateindustries in the near future.

SPEAR has been designed for extreme simplicity to reduce the number offatal errors the system may experience on its voyage to Europa. Thereare no large deployable mechanisms in the design, which reduces thepotential for failure within orbit significantly. The reactor is safe innature and only produces radiation via activation of control rods tobegin the nuclear fission process. This design reflects a simplisticpayload of CubeSats which may be deployed at Europa. SPEAR could bemodified to deliver a similarly sized payload to other locations withinthe solar system. Again, a system of CubeSats is preferred because ofits high technology readiness level (TRL) and industry familiarity withthe CubeSat form factor.

Electrical Power System

SPEAR PROBE 2400 comprises reactor and ATEGs 110 a-110 n. In oneembodiment, reactor comprises 15 kWt reactor coupled with ATEGs 110a-110 n to deliver 3 kW of electrical power. Electrical power may beapplied, at least in part, to Nuclear Electric Propulsion (NEP) system.By way of example only and not by way of limitation, SPEAR is describedbelow in connection with a mission to Europa. The following elements arelabeled in FIG. 24:

-   -   110 a-n: reactor    -   120: reflector    -   2402: heat pipes to radiators    -   2404: radiators    -   2406: structural truss    -   2408: Auxiliary propellant tanks    -   2444: Primary propellant tanks    -   2410: shielding    -   2422: mounting platform    -   2412: CubeSat deployment bays    -   2414: electric thrusters    -   2418: communications RF dish    -   2416: secondary RF dish    -   2420: mounting hardware

The Swarm Probe Enabling ATEG Reactor (SPEAR) system may venture to makedeep space exploration open to private industries. Several noveldesigns, including a nano-sized nuclear reactor and advancedthermoelectric generators (ATEGs), combine to produce one of the highestperforming deep space platforms in existence. The ATEGs used to harvestpower from the nano-reactor offer solid state energy productionefficiencies well above current technologies. This advancement, whichcan be applied to a multitude of space technologies, is what drivesSPEAR's potential for success. With a conservative 3 kW of electricalpower, SPEAR outperforms any previous deep space power system includingRTGs and solar panels (beyond Earth orbit) and contends with futurenuclear systems in development. SPEAR can explore vast stretches of thesolar system at fractions of the cost of traditional missions. Itscompact form factor and low mass allows it to be launched on inexpensivelaunch vehicles, and its nuclear electric propulsion system can propelit to the far reaches of the solar system. SPEAR has been designed toinclude a swarm of 10 CubeSats in a 6U form factor which can be deployedat its final destination or throughout its mission. The gradualminiaturization of spacecraft into CubeSats has shown that an immenseamount of high impact science can be placed in a small package. TheCubeSat design is familiar to universities, private industry, andgovernment organizations looking to test technologies and perform highimpact scientific studies. Instead of being limited to Earth orbit,SPEAR can transport CubeSats millions of kilometers away allowing manynew participants to explore deep space at a fraction of the cost oftraditional missions. SPEAR has the capability to bring new scientificresearch from deep space back to Earth, pushing humanity's explorationfrontier further than ever before.

One particularly exciting application for the SPEAR probe is theexploration of Jupiter's moon Europa. NASA has put a mission to exploreEuropa and investigate its habitability as the second highest-prioritymission for the decade after the Mars 2020 mission. Its expected missioncosts at the time of the survey was estimated at $4.7 billion. Thismission is expected to launch sometime in the 2020's with an unknownlaunch date and launch vehicle. While this mission contains many scienceinstruments, its costs and uncertainty in launch date make its futureuncertain. SPEAR could transport a swarm of CubeSats to Europa for afraction of the cost to accomplish more focused science goals. EachCubeSat may explore the Europa environment with high resolution cameras,detectors, and communications devices. With evidence of plumes eruptingfrom Europa's surface, the primary goal of the CubeSats may be to flythrough these plumes and analyze the material for evidence ofextraterrestrial life. This swarm has a higher probability ofintercepting plumes, maximizing the potential to discover organicmolecules. The high-resolution maps generated by onboard cameras may beparamount for future missions that intend to land on the surface ofEuropa. At Europa, the SPEAR probe may have more power available thanany other deep space probe allowing for large quantities of data to betransmitted to Earth for analysis. SPEAR has the capabilities to unravelthe mysteries that Europa contains and answer some of the greatestscientific questions.

The SPEAR spacecraft is an interplanetary nuclear electric propulsion(NEP) probe designed to study the Europa environment in search of life.This ultra-lightweight probe is meant to significantly reduce missioncosts for interplanetary space missions and open deep space explorationto private industries by using a majority of commercial off the shelf(COTS) components as well as a low enriched uranium (LEU) reactor. Witha total mass of 1517 kg, SPEAR can be launched at a relatively cheapprice compared to most interplanetary missions. With a comparativelysmall form factor, it may be able to ride share with another primarypayload or launch on board a small dedicated rocket. SPEAR's currentdesign utilizes limited moving parts to reduce failure modes when beingdeployed in space and has been designed to fit inside the Minotaur IVlaunch vehicle. Modifications including extendable booms and radiatorscan be used to further decrease the size of SPEAR to fit within evensmaller launch vehicles or be compact enough for a ride share mission.

Earth Orbit and Spiral Transfer

After launching into orbit around Earth at 700 km, SPEAR may engage thereactor control rods to begin the fission process. The reactor may notproduce radiation until this process begins, reducing radiationcontamination concerns on Earth. Once activated, the SPEAR may produce15 kW of thermal power and is estimated to produce 3 kW of electricalpower from the ATEG system. SPEAR may then utilize its NEP system toconduct a spiral transfer maneuver to escape Earth.

Orbiting around Earth and conducting the spiral transfer maneuverpresents the maximum thermal load SPEAR may see due to the Earth'salbedo, infrared signature, and proximity to the sun. To reduce thethermal load on the spacecraft, SPEAR may be slowly rotated around itslong axis to ensure even heating of the spacecraft. During the initialphases with such close proximity to Earth, ground controllers are ableto use this time to study the performance of the reactor and collect anyimportant science data transmitted by SPEAR before interplanetarytransit begins.

After leaving Earth's influence, SPEAR may continue on itsinterplanetary cruise phase to Jupiter. Cost models include staff tomonitor navigation to make minor course corrections on its journey tothe Jovian system, but subsequent efforts may be made to introduce anartificial intelligence-based guidance system. This may limit the needfor a full staff to be present during the somewhat long transit times,and human intervention may only be necessary in the rare event thatsomething unexpected happens to the craft while flying through space.

Interplanetary Cruise Phase

During this time, the reactor may continue to operate at full power forthe NEP system while its other systems enter a hibernation mode. Vitalcomponents of the spacecraft are protected from radiation coming fromthe reactor via a radiation shadow shield near the propellant tanks atthe rear section of the craft. This shadow shield is designed to protectthe most sensitive components of the spacecraft including electronicsand the CubeSat payloads. To minimize mass, the shield does not protectsome structural components and is positioned far away from the reactorwith substantial Xe propellant in the tanks also blocking some ofradiation. By not fully encapsulating the reactor with shielding,hundreds of kilograms of dead mass can be saved, and the radiationharmlessly dispersed into space. SPEAR may still be subjected togalactic cosmic rays (GCRs) and any solar particle events (SPE) that mayoccur, but the included shield and radiation hardened electronics mayassist with longevity during transit.

Europa/Jovian System Operations

Upon arrival at Europa, SPEAR may begin studying the Europa environment.SPEAR may contain only a few scientific instruments on the main craft,as most of the payload mass has been allocated to the CubeSats.Nonetheless, SPEAR may monitor the surface of Europa for several Europadays (approximately 3.5 Earth Days) to investigate changes to thesurface and gather preliminary data.

Each CubeS at can apply a small amount of delta-v to change its orbitalinclination and cover the maximum amount of area about the main orbit ofthe SPEAR craft. CubeSats may be launched two at a time from oppositesides of the SPEAR probe to minimize attitude changes imparted on SPEAR.Each CubeSat may be equipped with several science instruments to monitorthe Europa environment and include a mechanism to capture and analyzeplume particles. The purpose of these 10 CubeSats is to maximize theprobability of intercepting a plume particle. With all the CubeSatsdeployed, a wide swathe of the moon is covered due to the CubeSatsweaving up and down relative to the main craft. After deployment of allthe CubeSats, SPEAR transitions into a “mother-ship” mode fortransmitting, receiving, and processing data. Sections below detail theRF and laser options for beaming power to the CubeSats to keep themoperating for the duration of the mission. The SPEAR craft processes theinformation gathered from the CubeSats and may intermittently transmitdata back to Earth with its high gain antenna. Each CubeSat is expectedto operate for roughly 30 days before reaching its maximum dose ofionizing radiation. After the 30 day CubeSat operation is complete andall scientific data has been transmitted to Earth, SPEAR's health andsystems may be assessed to determine any additional missions that can becompleted before succumbing to radiation. So long as systems remainfunctional, the swarm can gather visual data on the surface, scan forlife, or monitor the local area.

The design for the nano-reactor involves lightweight moderator materialsand low enriched uranium (LEU) to minimize launch costs and allow forprivate ownership of the nanoreactor. A small array of batteries may beused to operate SPEAR may its nano-reactor is not operating. This shouldonly occur during initial operations around Earth and any situationswhere the reactor needs to be powered down.

FIG. 25 illustrates electrical subsystem of SPEAR, according to anexemplary embodiment showing an electrical diagram of an example ATEGpower production circuit capable of providing power and chargingbatteries. Batteries allow for reactor start up and contain enough powerfor 24 hours of emergency operation for ground crews. The SPEARElectrical Power System (EPS) utilizes the available energy from an ATEGcapable of being configured to supply two individual and unique loads.The EPS has two buses; a 3 kWe source of relatively-high DC voltage toserve as the energy source for the NASA developed NEXT-C electricpropulsion (EP) system; and a separate ˜100We source of power at avoltage suitable as input to a battery charge controller and regulatorsupplying power to the 28 VDC SPEAR spacecraft bus. Although SPEAR isshown and described as comprising two ATEG units, embodimentscontemplate SPEAR having two independent ATEG sources. Embodimentsfurther contemplate (and desirable from mass, volume and thermalmanagement perspectives) to provide both high power and low power tapsfrom a single ATEG unit.

Acronyms for SPEAR EPS Functional Architecture Diagram

Acronym Definition

ATEG Advanced Thermoelectric Generator Batt Battery

CHG Charge

Ctrl Control

EMI Electromagnetic Interference

EP Electric Propulsion

Ess Essential (Bus)

FET Field Effect Transistor

FLT Flight

GSE Ground Support Equipment

Htrs Heaters

kW Kilowatt

PCM Power (Subsystem) Control Module

PPU Power Processing Unit (for EP)

PWR Power

S/C Spacecraft

Sw Switched (Power Bus)

TBD To Be Determined

W Watt

TABLE 5 100 W_(e) 3 kW_(e) Current (A) 2.856 27.094 Voltage (V) 35.017110.722 Efficiency (%) ~30% ~35.5% Number of ATEG couples (#) 859 2,715ATEG mass (kg) 0.235 1.696 Configuration All in series All in series

TABLE 5 illustrates 100 W_(e) and 3 kW_(e) ATEG units not accounting forthermal shielding and other heat conversion hardware, raw thermoelectricmaterial only.

The EPS is designed to start up upon SPEAR deployment from the launchvehicle. An Activate Switch may close upon deployment, which wouldswitch on main spacecraft bus power and energize a field effecttransistor (FET) switch to also connect 28V main bus power to a“startup” 5 VDC converter. The 5V output of that startup converter maysupply system boot up power to the Power Controller Module (PCM), amicroprocessor (for simplicity not explicitly shown in the figure) thatthen would execute the software instructions necessary to turn on therest of the power system and to configure additional switched 28V busesto supply power to the rest of the spacecraft in a planned, controlledmanner. Some of those switched buses may need to be electromagneticinterference (EMI) filtered to ensure that electromagnetic noiseoriginating from a component on one bus would not propagate to deviceson other spacecraft buses.

Upon successful startup and self-checkout of the rest of the systems onthe spacecraft, all initially performed with power supplied byLithium-Ion battery modules, the ATEG may be activated. In at least oneexemplary embodiment, the batteries have been sized to operate for atotal of 48 hours, with a storage capacity of 108 Ah.

The activation may be via a motor driver signal from the PCM. Onceactive, the ATEG may source power to maintain the 28V main busindefinitely, to charge the battery (to be available as a futurecontingency power source), as well as to source high power for the EPpower programming unit (PPU), and thus the Hall Effect Thruster. Onceactivated, the ATEG may operate continuously. During those times whenthe EP is not consuming power, the 3 kWe ATEG may need to have its powerdissipated via a shunt resistor to prevent the ATEG from overheating.Connections to that shunt, as well as control of the EP PPU, may also bemanaged by the PCM.

Prior to launch, control of the spacecraft and charge management of thebattery may be possible via Ground Support Equipment (GSE). The GSE mayhave the ability to set main bus switches in a “safe for launch” OFFstate. Additional protection from inadvertent startup is provided by an“Arm for Flight” cable jumper that prevents any power from the batterybeing applied to the spacecraft 28V bus until the jumper is installed.Operationally, installation of that jumper would take place as one ofthe last activities during close-out operations just prior to launch.The GSE may also have the ability to source 28V spacecraft power via anindependent connection, which would only be used to check out spacecraftcomponents prior to launch. The PCM is inhibited from initiating itspost-deployment activation sequence by A-B lockout switches on the 5Vpower that are intended to be used to operate the PCM onlypost-deployment.

At Europa, SPEAR may have an unparalleled quantity of power for numerousapplications including science operations, powering of CubeSats, andcommunications with Earth. Each CubeSat may be powered via radiofrequency (RF) charging from the SPEAR Probe. This may minimize thecomplexity of the CubeSats and their systems to ensure mission success.The RF charging as attractive to charge the CubeSats due to the factthat a high power RF system is already installed and used forcommunications. Additionally, changing the reactor power levels can becomplicated due to fission product buildup and the presence of neutronpoisons in the reactor, so it is recommended that the power productionstay as constant as possible during the entire mission. As it mayalready be generating 3 kW of continuous electric power and has an RFsystem on board, using the RF communications system for charging seems aviable method for keeping masses low.

Another feature of the ATEG system is to provide the desired voltage andcurrent to different systems without changing the production profile ofthe power system. Various voltages and currents can be achieved bysimply changing which TEG feet are in series or parallel when the unitsare installed. Utilizing this unique feature, the power processing canbe more efficient as the need to convert between high voltage, highcurrent, and other sources can be reduced significantly. In the diagramof FIG. 25, a 100 W subsystem power source is wired separately from themain ATEG array and operates at 35V. The main array has more unitsconnected in series, which brings the total power up to 3 kW and 110V.

Communications

A prominent feature of SPEAR is its high gain antenna situated on thebottom of the spacecraft. This antenna may be used to communicate withEarth throughout its mission. Instead of powering down the reactor atEuropa to minimize thermal requirements, SPEAR may utilize the powerfrom the reactor to beam power to the CubeSats and send science databack to Earth. The 1.5 meter diameter dish is comparatively smaller thanother spacecraft such as Juno or Cassini with 2.5 meter and 4 meterdiameter dishes respectively. The high gain antenna size is restrictedby the fairing size of the launch vehicle, so a 1.5 meter dish waschosen to offer high performance with minimal form factor. AssumingSPEAR broadcasts at similar frequencies to Juno on the fifth and sixthchannel of the deep space network, the respective gains from SPEARshould be 33 dB and 32.5 dB. When Jupiter is at its farthest locationfrom Earth, the received power should be −136.41 dBm and at its closestlocation the received power should be −133.02 dBm. These values appearto be between that of the Juno probe and Cassini with received powers of−129 dB m and −137.87 dBm. The large amount of power available forcommunications makes it possible to reduce the size of the antenna dish,allowing SPEAR to fit within smaller payload fairings compared to otherdeep space missions.

SPEAR may not only need to communicate with Earth, but also relayinformation to and from the 10 deployed CubeSats. S-band receivers,transmitters, and patch antennas may be utilized to communicate withSPEAR. Most communications should occur between SPEAR and individualCubeSats, but the s-band communications architecture allows for highdata rate communication between satellites. This could prove useful ifSPEAR needs to send information to CubeSats that are not reachable fordirect communication.

Propulsion

Interplanetary NEP systems have no flight heritage, but many systemshave been proposed for missions to the outer solar system. Most of thesesystems involve dynamic power generation systems for their highconversion efficiency. The Jupiter Icy Moons Orbiter (JIMO) was a 100 kWNEP spacecraft slated to explore three of Jupiter's moons includingEuropa with a Brayton power conversion system. This massive spacecraftwas eventually cancelled, and the concept never fully tested. The SPEARprobe leverages similar concepts from the JIMO mission, including theuse of a high specific impulse ion thruster. SPEAR may utilize the NASAEvolutionary Xenon Thruster (NEXT), or a system like it, for its primarypropulsion. A NEXT thruster is capable of 4,190s of specific impulsewith projected total throughput of over 730 kg of propellant. Theseparameters make the NEXT thruster more than ideal for the 10 yearmission lifetime.

Thermal Control

Positioned on the long axis of the SPEAR probe are four large radiators.These radiators are used to dissipate the 12 kW of waste heat from thereactor. Heat pipes are used to transfer heat from the back of the ATEGsout to these radiators. The use of four radiators results in a lesseffective design due to the radiators shining on each other, but due tothe truncated space of the launcher fairing it was necessary to achievethe required surface area. The possibility remains to deploy radiatorsor extend a telescoping boom for the main shaft, but this concept ofextending the boom would introduce moving parts and potential modes offailure. For the current design, the extra radiator mass was acceptablebut other missions may be willing or able to use a more optimizedsystem.

A detailed analysis of the radiator system was conducted to determinethe maximum heat rejection while maintaining a low mass system. With acomplex trajectory, SPEAR's radiators need to reject heat as SPEARorbits Earth, on its journey to the Jovian system, and while SPEAR is atEuropa. A carbon-based material with heat pipes containing water as theworking fluid were chosen. Analysis was performed on the trajectorySPEAR would take to the Jovian system to determine the optimal size ofthe radiators to maintain a steady 350K cold side for the ATEGs.

Attitude Determination and Control System

As SPEAR travels through space, it may utilize its electric propulsionsystem to perform various attitude control maneuvers as well as a suiteof attitude control thrusters. Four reaction control wheels may be usedto control SPEAR's attitude in orbit as well. These systems were chosenas they primarily rely on power generated by the spacecraft instead ofadditional propellant systems to minimize mass and complexity, and theSPEAR craft has a surplus of electrical power. With the excess poweravailable at Europa, the reaction wheels may be used to the fullestextent when attitude changes are required by the SPEAR Probe. Thereaction wheels are better suited for use around Europa as this is whenthe system may be at its lightest. Around Earth orbit, the spacecraftmay rely more on its thrusters to provide gradual attitude changes inaddition to the reaction control wheels. Several sun sensors, startrackers, and magnetometers may be used to determine the attitude of thespacecraft. The star trackers may be especially useful while navigatingto Europa and other deep space locations. Several of these componentsare already commercially available and rated to moderate radiationlevels, which may minimize the amount of shielding required in theEuropa environment.

Reaction wheels and attitude control thrusters may be used around Earthand at the Jovian system to desaturate reaction wheels as well asperform attitude correction maneuvers. In deep space, a majority of theattitude control may be accomplished through the use of the primaryelectric propulsion system. This system may make gradual changes to thespacecraft's attitude for any corrections in its trajectory.

The monopropellant control system in at least one exemplary embodimentcomprises 22 kg of monopropellant and 12 thrusters to provide agilemaneuvering of the spacecraft and in deep space. This may be used forADCS corrections or maneuvers that are too quick for the reaction wheelsand passive system to support.

Command and Data Handling

SPEAR may use the previously demonstrated and highly reliable RAD750computer to process information and handle data. This computer hasfantastic heritage with numerous deep space missions. Included in thenumerous probes that use the RAD750 computer is Juno, which is currentlyorbiting Jupiter. The RAD750 is capable of withstanding up to 1 Mradtotal ionizing dose (TID), which is ideal for surviving the radiationfrom the reactor and the radiation environment from Europa. While theRAD750 comes in 3U and 66U form factors, it was not considered for theCubeSat computers because of its mass and power consumption.

Payload

The primary payload aboard SPEAR is the array of 10 CubeSats that may bedelivered to Europa and form a swarm around the moon. Contrary to asatellite constellation in which all pieces remain in a constantposition relative to each other, the CubeSats may be deployed atdifferent times to create a dynamic weaving pattern for maximum surfacecoverage. Thus, the cluster of satellites may more resemble a swarm ofinsects than a constellation of stars. In at least one embodiment, eachCubeS at has a mass of 7 kg for a total payload mass of 70 kg. Morespecifically, each CubeSat may contain a multispectral camera, Ramanspectrometer, lab-on-a-chip for chirality detection and imaging via amicroscope. The latter two may be used to detect the composition ofEuropa's plumes and detect for any extraterrestrial life. CubeSatscientific payload and other systems may be described in more detail inanother section.

The secondary payload for Europa mission may comprise severalobservational instruments to observe Europa's surface. With the primarypayload looking for signs of life, SPEAR may assume a primarilyobservational role of Europa. Reuse of several systems from other deepspace probes is considered advantageous to reduce development costs. TheJunoCam produced by Malin Space Science Systems may be used onboard theSPEAR probe to image the moon's surface. This technology has alreadyproven itself onboard the Juno spacecraft and is capable of withstandingthe extreme Jovian environment. Infrared cameras may also be included onthe SPEAR probe monitor the temperature levels of Europa's vast icesheet. SPEAR may also closely monitor the perturbations experienced bythe CubeSats during its mission and relay any useful science data aboutanomalies around Jupiter. The SPEAR probe may also contain two sets ofthe same life detection instruments as the CubeSats contain for missionredundancy. After the primary CubeSat mission is over, missionextensions may be investigated based on the health of the spacecraft andpropellant remaining.

Mass Model

A detailed mass model of the SPEAR system was created to estimate theinitial wet mass of the system. SPEAR's original mass estimate was 1100kg, but, after in-depth analysis the total mass increased to just over1,500 kg. A major factor in this increase was due to the size of theradiator system and the fact that 4 radiators were required to fit inthe launch fairing. A cold side temperature of 350K was originallychosen to provide a comparison to the MMRTG systems available now.However, increasing this temperature would decrease the radiator areabut decrease ATEG efficiencies. Additionally, expandable booms or alarger fairing would allow for more efficient radiator operation andreduced total mass.

TABLE 6 Section Quantity Mass/Unit (kg) Mass (kg) Reactor system 134.9Core 1 58.4* 58.4 Fuel 1 76.3* 76.3 Control rods and actuators 1  0.2*0.2 Shield 32.0 Main power conversion 11.9 Heat exchanger (hot) 1  7.5*7.5 ATEGs 1  4.4* 4.4 Thermal control system 200 Radiators 4 50.0* 200.0Primary propulsion system and Electrical 59.5 Bus Thruster cluster 113.5 [14] 13.5 PPU 1 40.0 [14] 40.0 Mount 1 6.0 [14] 6.0 Secondarypropulsion system 35.1 Thruster Clusters 1 17.5 [15] 17.5 Reactionwheels 4 4.4 [16] 17.6 Auxiliary power system 22 Batteries, Drivers 112.0* 12.0 Cables 1 10.0* 10.0 Structures 24.8 Main truss 1 20.8* 20.8Tank and payload dispenser support 4  0.8* 3.1 Secondary Structures 1 0.9* 0.9 Avionics 25.7 Star trackers 2 0.35 [17] 0.7 High-gain antenna1 15.0* 15.0 Structures and shield 1 10.0* 10.0 Tankage 41.4 Missionpropellant tanks 4  6.4* 26.4 Feed lines and valves 1 10.0* 10.0Insulation 1  5.0* 5.0 Payload and provisions 103.09 Board computer 20.15 [12] 0.29 Telecommunication system 1 16.8* 16.8 Payload dispenser10 1.6 [18] [19] 16 Cubesats 10  7.0* 70.0 Mission propellant mass 804.3Reaction control propellant 22.0

TABLE 6 shows a mass budget of the SPEAR probe, according to anexemplary embodiment. Another significant contributor to the overallmass is the amount of Xe propellant onboard the system. Withimprovements to the trajectory, the total mass of the system can besignificantly decreased. As the ATEG system is further investigated andthe possible efficiency for this system more refined there could be vastimprovements upon the mass. A 20% ATEG efficiency was used asconservative estimate, and improvements to this decrease the amount ofheat that needs to be rejected and increases the power available to thepropulsion system, which would translate to a higher specific impulse.This higher specific impulse would correspond to a drop in the totalpropellant required for the mission.

Reference in the foregoing specification to “one embodiment”, “anembodiment”, or “some embodiments” means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment of the invention. The appearancesof the phrase “in one embodiment” in various places in the specificationare not necessarily all referring to the same embodiment.

While the exemplary embodiments have been shown and described, it may beunderstood that various changes and modifications to the foregoingembodiments may become apparent to those skilled in the art withoutdeparting from the spirit and scope of the present invention.

1. A thermoelectric converter comprising: a thermoelectric generator; aradiation source; wherein the thermoelectric generator includes a hotsource, a cold source, n-type material, and p-type material; wherein theradiation source captures neutrons for producing ionizing emission usingstable isotopes or fissile atoms and emits ionizing radiation thatincreases electrical conductivity.
 2. The thermoelectric converter ofclaim 1 wherein the radiation source is external to the thermoelectricconverter.
 3. The thermoelectric converter of claim 2 wherein theexternal source is a reactor. 4.-5. (canceled)
 6. The thermoelectricconverter of claim 1 wherein the thermoelectric converter uses materialsthat respond to the radiation source by changing material properties. 7.The thermoelectric converter of claim 6 wherein the materials are notmetal.
 8. The thermoelectric converter of claim 6 wherein the changingmaterial properties are from one of: radiation induced conductivitychanges to electrical conductivity, changes to thermal conductivity, orchanges to Seebeck coefficient.
 9. The thermoelectric converter of claim6 wherein the changing material properties take place over a specificrange of temperatures.
 10. The thermoelectric converter of claim 1wherein the radiation source uses alpha, beta, gamma, x-ray, orneutronic radiation.
 11. A method of using radiation to reach highefficiency with a thermoelectric converter, the method comprising:providing a thermoelectric generator and a radiation source, wherein thethermoelectric generator includes a hot source, a cold source, n-typematerial, and p-type material; emitting ionizing radiation with theradiation source to increase the electrical conductivity which stripselectrons in the n-type material, the p-type material, or both then-type material and p-type material from their nuclei with the electronsthen free to move within the material.
 12. The method of claim 11wherein the radiation source is an external source.
 13. The method ofclaim 12 wherein the external source is a reactor.
 14. The method ofclaim 11 wherein the radiation source is an internal source.
 15. Themethod of claim 14 wherein the internal source is a radioisotope dopant.16. The method of claim 11 wherein the thermoelectric converter usesmaterials that respond to the radiation source by changing materialproperties.
 17. The method of claim 16 wherein the materials are notmetal.
 18. The method of claim 16 wherein the changing materialproperties may be from radiation induced conductivity changes toelectrical conductivity, changes to thermal conductivity, or changes toSeebeck coefficient.
 19. The method of claim 16 wherein the changingmaterial properties take place over a specific range of temperatures.20. The method of claim 11 wherein the radiation source uses alpha,beta, gamma, x-ray, or neutronic radiation.
 21. The thermoelectricconverter of claim 1 wherein the stable isotopes are boron or lithium.